Is your female Chihuahua desexed? If she is, she may not have the tolerance towards a pup's playfulness that an intact bitch might. I introduced a pup here and my adult cattle dog, not speyed, thought he was wonderful, started acting like this was her pup she had to rear and educate. LOL. I think you'd have to monitor the situation. One advantage could be that if you want to travel, but leave the dogs at a boarding kennel, they would be company for each other while you are away. Like Hazel, I went from having one to 2, to 4 and on up to 12 with no regrets. I think I'll hate it when eventually I get back down to only two.